Tides, and Why They Differ So Much by Place

The basic mechanism: two bulges, not one

Tides are caused by the gravitational pull of the moon, and to a smaller degree the sun, acting on the water in the oceans. The simplified textbook picture is two bulges of water, one on the side of Earth facing the moon, where its pull is strongest, and one on the far side, where Earth itself is pulled toward the moon slightly more than the water on that far side is, leaving a bulge there too. As Earth rotates through these two bulges roughly once a day, most coastlines experience two high tides and two low tides within about 24 hours and 50 minutes, the extra 50 minutes accounting for the moon's own slower orbit around Earth. This is why tidal cycles track the lunar day rather than the 24-hour solar day, and why high tide at a given beach shifts to a slightly later clock time each day.

Why the open ocean is not where tidal range gets extreme

If the ocean were a single, uniformly deep basin, tidal range would be fairly similar everywhere, roughly a metre or so of difference between high and low water. In practice, tidal range varies from a few centimetres in places such as parts of the Mediterranean and Baltic seas to more than ten metres in a handful of locations worldwide, and that variation comes almost entirely from the shape of the coastline and the sea floor, not from the moon pulling harder in some places than others. Tides behave like waves travelling through ocean basins, and the shape of a basin, bay or channel can amplify or dampen that wave the way the shape of a container changes how water moves when it is disturbed. A funnel-shaped bay that narrows and shallows toward its head, matching the natural resonant period of the incoming tidal wave, can amplify the tide dramatically; a wide, open, relatively straight coastline usually does not.

What tidal range means for a stretch of sand

Tidal range describes the vertical distance between predicted high and low water at a given location, and it directly determines how much of a beach is submerged versus exposed over a tidal cycle. On a beach with a large tidal range and a gently sloping profile, the horizontal distance between the high-tide line and the low-tide line can be hundreds of metres, meaning the beach looks like an entirely different place a few hours apart. Playa de las Catedrales in Galicia, Spain, demonstrates this clearly: a corridor of rock arches up to thirty metres tall, cut through slate and schist cliffs, is walkable sand only at low tide and is submerged as the Cantabrian Sea returns, which is also why regional authorities require a timed permit system there to manage visitor numbers around the tidal window. Rhossili Bay on Wales' Gower Peninsula shows a related effect on a smaller, more localised scale: Worm's Head, a tidal island at its southern end, is connected to the mainland by a causeway that is only exposed and crossable on foot for a window of a few hours either side of low water, and is cut off by the sea the rest of the time.

Spring tides, neap tides and the sun's smaller role

Tidal range at a single location also varies on roughly a two-week cycle because of the sun. When the sun, moon and Earth are roughly aligned, at new moon and full moon, the sun's gravitational pull adds to the moon's, producing the largest tidal ranges of the month, called spring tides (the name refers to the tide "springing up," not the season). When the moon is at a right angle to the sun relative to Earth, around the first and last quarter moon, the sun's pull partially cancels the moon's, producing the smallest tidal ranges of the month, called neap tides. This cycle means the same beach can show a noticeably different high-tide line from one week to the next even with no other change in conditions, which is one reason agencies that publish tide predictions, such as NOAA's Center for Operational Oceanographic Products and Services, issue them as a continuously updated schedule rather than a single fixed number.

Why this matters for understanding a beach

Tidal range shapes a surprising amount of what a beach is physically like: how wide its exposed sand is at a given hour, whether tide pools and reef flats are reachable on foot or only by boat, whether a river mouth or lagoon changes character twice a day, and how far storm waves can reach when they coincide with an already high tide. Reading a beach's typical tidal range, available from regional tide tables and government oceanographic agencies for virtually any coastline, explains a great deal of the hour-to-hour difference in how the same stretch of sand can look and behave.